Ronald Hanson at FiO+LS

Ronald Hanson, giving Wednesday plenary talk at FiO+LS 2019. [Image: OSA]

In the vast space of possibilities cited for the impact of quantum technology, the idea of a “quantum internet” offering new levels of security and functionality holds particular allure. In the Wednesday plenary talk at Frontiers in Optics+Laser Science 2019, Ronald Hanson, the director of the QuTech project at the Delft University of Technology, Netherlands, sketched out some of those possibilities, and how his team at Delft and others are working to make this vision a reality.

Context from the past

Hanson called the drive for a quantum internet “an amazing endeavor … but also a challenging one,” and a long-term goal that is “not going to happen in the next decade.” To offer some perspective, however, he cast the audience’s gaze back to a moment almost exactly five decades ago, on 29 October 1969, when the first point-to-point attempt was made to log in remotely across the network that eventually became the modern (classical) internet.

As is well known in computer-science circles, the system crashed after only the first two letters of “login”—“lo”—had been sent. Yet however inauspicious that beginning seemed, Hanson observed, “something very fundamental happened here—a key step in the development of the internet.” The implication: the current state of quantum internet research may provide little idea of where it could ultimately go.

In an analogous way, Hanson is working both with European partners and in projects centered in the Netherlands to take the initial steps toward a quantum internet. In his talk, he focused on the work taking place in Delft, and on his lab’s effort to build “the first entanglement-based internet,” a four-node network that would connect centers in Delft, Leiden, The Hague, and Amsterdam.

Step by step

But what is a quantum internet? In its most evolved sense, Hanson defined it as a system that would “enable the exchange of qubits between local processors anywhere on earth—an analog of the classical internet, with quantum information.” Such a goal, he said, is many years away—but there are intermediate steps on the path to such a general quantum computing network that constitute lower-hanging fruit, but that could still offer useful applications.

Steps toward a quantum internet

  • 5. Quantum computing networks
  • 4. Fault-tolerant few-qubit networks
  • 3. Quantum memory networks
  • 2. Entanglement distribution networks
  • 1. Prepare-and-measure networks
  • 0. Trusted repeater networks

Right now, Hanson said, outside of the laboratory, the only demonstrations have been at the bottom rung of this ladder: trusted repeater networks, which he said involve “point to point connections, stitched together in a classical way.” At a somewhat higher level, systems of “entanglement distribution networks”—in which entangled qubits are exchanged across the network, but without any quantum memory to preserve that state—could nonetheless enable device-independent security protocols and, thus, the exchange of quantum security keys using devices that require no trust.

Still further up the ladder are “quantum memory networks,” in which the entanglement swapping is accompanied by some quantum memory to maintain quantum states. This kind of system, Hanson said, allows users to “distribute quantum entanglement and also do some processing on it.” It could enable cloud computing in which the nature of the underlying data is hidden from the processing server—a security feature not possible in the classical internet.

The path to a multi-node demonstration

The four-city quantum internet demonstration that Hanson and colleagues are attempting to build in the Netherlands would be such a quantum memory network. The effort faces some steep and familiar challenges: creating methods for generating entanglement between nodes at a rate faster than the rate entanglement is lost; robust storage of quantum information; and the ability to bridge long distances, by converting photons to telecom wavelengths ready for fiber transmission without destroying their entanglement.

In his plenary, Hanson reviewed some of the progress that his team at Delft has made on all three fronts. His team, he noted, is focusing on a specific qubit platform, electron spins in diamond nitrogen–vacancy centers—so-called color centers in diamond, in which a carbon atom in the crystal lattice has been replaced by a nitrogen atom and a lattice vacancy.

Hanson said the team’s goals for next year were to build the first multimode network in the lab, and to demonstrate communication of entanglement between nodes in Delft and The Hague—two of the four cities in the targeted Netherlands network.

Demonstrating the quantum advantage today

Eleni Diamanti

Eleni Diamanti. [Image: Photo by Claire Gaby]

In addition to his team’s work in the Netherlands, Hanson’s group is part of the European Quantum Internet Alliance (QIA), a multi-country effort funded under the E.U. Quantum Flagship program to make progress on a variety of fronts toward a quantum internet. After Hanson’s plenary, OPN talked with another QIA participant, Eleni Diamanti of CNRS and Sorbonne University, Paris, France, about other efforts in this space.

Some of Diamanti’s own work, which she covered in an FiO+LS talk of her own later in the day, focuses on “showing the quantum advantage” for networking applications, in “a more near-term sense” than the work Hanson focused on. “The key point in going toward more applicable stuff is matching the experimental configuration you have with use cases,” she said. “What do people want to do with the quantum network? What security do they need?”

This kind of application viewpoint, she said, is “very complementary” to the longer-term effort that Hanson is working on, and aims at demonstrating how quantum networks can start to benefit secure communications with technology that already exists or will soon be developed. When a “full-fledged” quantum internet comes into being, “we will find even more applications,” Diamanti said. But from the point of maintaining interest and forward momentum, it’s also important, she suggested, to show how the technology can be useful now or in the near future.

European infrastructure project ahead

Diamanti also is enthused about the next step in the E.U.’s support of a quantum internet, the European Quantum Communication Infrastructure (QCI). This decade-long program, announced in the spring of this year, aims to build on the progress of the Flagship-funded QIA and other efforts, to build Earth- and space-based quantum communication technology and bring it closer to commercial use.

While the details of QCI implementation have yet to be fleshed out, Diamanti believes it signals a significant commitment. “There’s going to be a big follow-up in Europe” on the quantum internet, she predicted, “spanning a whole range of technologies.”